Electrical bushing having means for lowering the power factor

ABSTRACT

A dry type bushing having an insulator provided with an axially extending bore, the wall of which is coated with a semiconductive glaze, is provided with means for shunting the air gap therein. A multitude of connections between the axially extending conductor and the semiconductor glaze are distributed both axially along the length of the bushing and angularly about the circumference of the wall of the bore. This distribution of a multitude of connections minimizes watt loss by placing the charging current directly in the region of highest electrical stress; and also, it distributes the current over a wide surface of the semiconductive glaze.

United States Patent 1 Stone 211 Appl. No.: 298,410

[52] U.S. Cl. 174/142, 174/152 R [51] Int. Cl. H011) 17/26, HOlb 17/42 [58] Field of Search 174/11 BH, 12 BH,

174/14 BI-1,15 BH,16 BH, 18, 31 R, 31.5, 73 R, 73 SC, 75 R, 75 D, 140 R, 140 C, 142, 143,152 R, 153 R, 167

[5 6] References Cited UNITED STATES PATENTS 1,661,823 3/1928 Hawley 174/140 C X 1,977,957 10/1934 Stevens 174/142 1,978,649 10/1934 Roberts 174/142 2,281,173 4/1942 SantOmIen 174/140 C X 3,055,968 9/1962 Spiece 174/31 R 3,086,073 4/1963 Arone et al 174/142 FOREIGN PATENTS OR APPLICATIONS 897,128 5/1944 France 174/31 R [4 Oct. 23, 1973 6/1935 Great Britain 174/142 430,076 777,514 6/1957 Great Britain.... 174/12 BH 320,903 5/1957 Switzerland 174/142 Primary Examiner-Laramie E. Askin Attorney-Robert C. Jones et al.

[ ABSTRACT A dry type bushing having an insulator provided with an axially extending bore, the wall of which is coated with a semiconductive glaze, is provided with means for shunting the air gap therein. A multitude of connections between the axially extending conductor and the semiconductor glaze are distributed both axially along the length of the bushing and angularly about the circumference of the wall of the bore. This distribution of a multitude of connections minimizes watt loss by placing the charging current directly in the region of highest electrical stress; and also, it distributes the current over a wide surface of the semiconductive glaze.

8 Claims, 5 Drawing Figures United States Patent 1 [111 3,767,843 Stone Oct. 23, 1973 IHHIIHH 9 ff-Serniconducfive glaze ELECTRICAL BUSHING HAVING MEANS FOR LOWERING THE POWER FACTOR BACKGROUND OF THE INVENTION Power factor measurements are made on bushings, separately and in combination with their related equipment, such as transformers, circuit breakers, etc., to assess the condition of the insulation.

In order for the power factor readings of a large piece of equipment or system to be of any meaningful value, the readings of the separate components when taken apart from the system must be both stable and of the same order of magnitude as the same readings taken on the other components of the system. Stable meter readings imply that the RIV or corona inception level must be above the test voltage level. Low meter readings are a function of construction and materials of the device.

Power factor readings are stable and generally below 5% in bushings having a construction which incorporates an oil filled, solid uniform insulation, encapsulation or condenser grading design or any combination of these methods. Any one or any combination of these methods can be used to equalize the electrical field or stress across the gap of the insulation system of the bushing and thus insure a high corona inception level and hence, stable power factor readings. The materials used, as well as the construction, helps to assure lower power factor readings.

Dry type bushing construction has none of the above design features, but is preferred for some voltage ratings because of its low cost and simplicity. However, due to the nonuniform electrical field accentuated by different dielectric constants of the air and porcelain insulation used in the construction, internal corona may result, leading to both high and unstable power factor readings unless special means are provided to make the field more uniform, or to eliminate and/or shunt air spaces of high corona inception probability.

When a bushings major insulation is made up of two or more different materials with greatly different dielectric constants, such as a solid bushing whose insulation is porcelain and air, the electrical stress across the various insulations is in proportion to the inverse of the dielectric constant and hence, can be very nonuniform over the entire insulation system. This nonuniformity can lead to corona discharge in the resulting higher stress, area.

In a solid bushing using air and porcelain as the insulation system, for example, corona discharges can be controlled by shunting out the medium which has the lowest dielectric constant, air in this case, thereby removing it from the insulating system and relying entirely upon the medium with the higher dielectric constant, porcelain. Although the resulting porcelain insulation gap is smaller than the air-porcelain system, the dielectric stress is uniformly distributed resulting in a lower voltage gradiant in the porcelain gap than the airporcelain system had in the air gap.

The power factor ofa bushing is a combination of the measurement of the charging and leakage current within the bushing and the watt loss associated with this current.

The majority of the charging current is directed to the area of highest electrical field which is located in the vicinity of the grounded flange. If the charging current must follow a high resistance path, there will result a large watt loss which will, in turn, distort the overall interpretation of the readings.

A semiconductive glaze or coating, is by definition, a high resistant coating. If the construction of the bushing provides a contact point between the bushing conductor and the coating at either end of the bushing where it can most conveniently be made, the majority of the charging current must travel through the high resistant path for approximately one-half the length of the bushing to the grounded flange area and the power factor reading which results can be much greater than is otherwise normal.

SUMMARY OF THE INVENTION In accordance with the preferred embodiment of the present invention, an insulating member, such as a ceramic insulator, is provided with an axial bore the wall of which is provided with a semiconductive glaze that is preferably fired into the wall surface of the bore so as to be permanently bonded to the insulator. Means for shunting the air gap comprising a multitude of connections between the axially extending conductor and the semiconductor glaze are distributed both axially along the length of the bushing and angularly about the circumference of the wall of the bore. This distribution of a multitude of connections minimizes watt loss by placing the charging current directly in the region of highest electrical stress, and also, it distributes the current over a wide surface of the semiconductor glaze.

It is accordingly a general object of the present invention to provide a dry type bushing having an air gap between the conductor and the insulator with means to maintain the bushing at a low power factor.

Still another object of the present invention is to provide a semiconductive glaze in combination with a suitable contact arrangement to shunt out the air gap in a dry type bushing.

Yet another object of the present invention is to provide a dry type bushing having contacts attached to the bushing conductor directly in the area of the grounded flanges to thereby shunt the long high resistance path.

Another object of the present invention is to provide a dry type bushing with means wherein the charging current directed to the ends of the bushing is made to follow a path from the center region of the bushing to the ends thereof through the high resistance coating so that the charging current, which is of a much lower magnitude, will thereby contribute much less to the watt loss of the system.

Still another object of the invention is to provide a dry type bushing with a novel contact arrangement to distribute the charging and loss current over the surface of a high resistance shield so as to minimize the losses due to current flowing through the shield. I

A further object of the present invention and advantages thereof will become more readily apparent from the following detailed description taken in conjunction with the drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view of the dry type bushing partly in elevation and partly in vertical section showing a preferred embodiment of the invention;

FIG. 2 is a developed view of a contact that is interposed between the wall of the insulator and the conductor shown in FIG. 1;

FIG. 3 is an enlarged fragmentary exploded view of the bushing shown in FIG. 1 showing the relationship of the several parts;

FIG. 4 is a developed view of a modified form of contact; and

FIG. 5 is a section through a bushing showing the assembly of the modified contact in relation to the conductor and the wall of the bore of the insulator.

DESCRIPTION OF THE INVENTION As previously mentioned, a dry type bushing construction has none of the design features of the oil filled or solid uniform insulation or encapsulated or condenser type grading design but are preferred for some voltage rating because of its low cost and its simplicity. However, due to the nonuniform electric field accentuated by different dielectric constants of the air and porcelain insulation used in the construction, internal corona may result, leading to both high and unstable power readings unless special means are provided to make the field more uniform, or to eliminate and/or shunt air spaces of high corona inception probability.

Tests were made to determine if the concept of shunting the air gap in a dry type bushing was practical ence of the bushing bore within practical limits. A preferred construction which optimized the findings was established comprising a pair of contact sheets each having a multitude of contact points evenly distributed around the circumference of the insulator bore and in an axial region centered around the grounded flange of the bushing.

As shown in FIGS. 1 and 2, a preferred construction of a dry type bushing 10 comprises a tubular insulator body 11 having external sheds 12 provided thereon to increase surface creepage distance. Immediately below the sheds 12 a mounting flange 13 is arranged and bonded in position with a suitable bonding agent. Flange 13 is provided with a plurality of spaced openings 14, one of which is shown, that serve to receive suitable bolts for securing the bushing to a device (not shown) such as a circuit breaker. A semiconductive glaze 15 is provided on the wall surface of the bore 16 of the insulator 11. Preferably the resistance of the semiconductive glaze 15 should be as low as practical within the state of the art. The semiconductive glaze 15 not only serves to aid in controlling power factor but also in controlling a corona effect experience. It is also preferred that the semiconductive glaze 15 be fired into the surface of the ceramic or porcelain insulator 11 so it becomespermanently bonded to and an integral part of, the insulator.

Disposed within the bore 16 of the insulator is a relative rigid conductor 17, the ends 21 and 22 of which are threaded and extend outwardly of the insulator. In use, the lower extending end 22 of the conductor must be centered with respect to the lower end 16A of the bore 16. This is true because the end 22 of the conductor 17 must be positioned with respect to another device (not shown) such as an interrupter and little tolerance variation is permitted. However, the upper extending end of the conductor 17 is normally attached to a somewhat flexible cable (not shown) and thus the upper end of the conductor 17 need not be centered with respect to the upper end 168 of the bore 16. However, in the manufacture of the porcelain insulators 11 it is virtually impossible to provide an insulator having a bore of which the walls thereof are symmetrical with respect to the axis of the insulator along the entire axial distance. Thus,'the bore 16 provided in the insulator 11 is of a diameter sufficiently large enough to accommodate the diameter of the conductor 17 and still provide an adequate air gap 23 between the conductor and the wall of the bore 16. I

In a preferred embodiment, the diameter of the bore 16 is 1.625-inches in diameter with the wall of the bore along its axial length being such as to pass a 1.375 inch diameter rod full length of the bushing. Thus, with this relationship obtained the conductor 17 which preferably has a diameter of 1.25 inches would have an adequate air gap 23.

To effect the centering of the lower end 22 of the conductor 17 with respect to the outer end 16A of the bore 16, it is desirablev that the axial end face of the lower end26 of the insulator 11 be provided with a spherical surface 25, the radius of which is substantially less than the length of the insulator. To secure the lower end 22 of the conductor 17 in centered relationship within the outer. end 16A of the bore 16 a cap 27 is provided. The cap 27 is provided with the threaded axial bore 28 and is engaged on the threaded end 22 of the conductor 17; The end face of the cap 27 adjacent to the spherical end surface 25 of the insulator 11 is provided with a radially extending flange portion 29, the surface of which is formed with a spherical surface 31, to complement the spherical surface 25 of the insulator 11. With this arrangement, engagement between the spherical surface 31 of the cap27 and the spherical axial end surface 25 of the insulator will effect a radial positioning of the end 22 of the conductor 17 into a centered position with respect to the outer end 16A of the bore 16. The complementary spherical surfaces 25 and 31 of the lower end of the insulator 26 and the end cap 27, respectively, form a ball and socket joint, the radius of which is substantially less than the length of the insulator 11. Accordingly, any radial displacement of the lower end 22 of the conductor l7is impossible because it cannot move about the center point of the ball and socket but rather is forced by the physical arrangement of the assembly to move about a center point at the opposite end of the insulator.

Another and more easily accomplished way of assuring the centering of the lower end 22 of the conductor 17 within the lower end 16A of the bore 16 is to provide the end of the insulator 11 with a conical surface having an angle A. Under this condition, the complementary surface 31 of the end cap 27 will be provided with a conical surface having an angle B, which is complementary to the angle A of the insulator end surface. 7

In the depicted illustration of bushing l0, the particular insulator 11 is substantially 26 inches long. The diameter of the lower flange 29 of the end cap 27 is three (3) inches and the angles A and B are substantially 12 degrees.

Accordingly, any radial displacement of the lower end 22 of the conductor 17 is impossible because the conductor cannot move about the center point of the ball and socket, but is forced by the physical arrangement of the assembly to move about a center point at the opposite end of the insulator.

In the manufacturing of porcelain insulators, such as the insulator 11, it is practically impossible to provide an insulator having a bore wall equidistant from the axis of the bore along the entire axial length thereof. Since the conductor cannot be bent to accommodate the distortion in the wall of the bore and must be maintained clear of the wall so as not to impart stresses to the insulator, the upper end 21 of the conductor 17 must be allowed to assume a position anywhere within the upper outer end 16B of the bore 16 so that the axis of the conductor is not distorted out of a straight line path. To allow the upper end 21 of the conductor 17 to assume any position within the confines of the opening 16B of the bore 16, the upper axial end face of the insulator 11 is provided with a spherical surface 37, the radius of which is substantially equal to the length of the insulator.

For securing the upper end 21 of the conductor in its assumed position an end cap 38 having an axial threaded bore 39 is threadedly engaged in the end 21 of the conductor 17 and is firmly engaged with the spherical surface 37 of the insulator. The surface 41 of the end cap 38 is formed with a spherical surface which is complementary to the spherical surface formed on the end 37 of the insulator 11. Thus, any radial displacement of the upper end 21 of the conductor 17 is impossible since the center point of the upper ball and socket joint is substantially at the opposite or lower end of the insulator 11 which is the same point about which the'overall insulator assembly forces the conductor 17 to move.

As in the case of the lower end of the insulator 11, a more facile manner of providing a ball and socket upper end is to form upper end surface 37 as a conical surface having an angle C. In this case, the complementary surface 41 of the end cap 38 will be formed with an angle D complementary to the angle C. The angles C and D of the surfaces 37 and 41 are chosen at a value to allow the upper end 21 of the conductor 17 to be stable in any radial position within the confines of the upper end opening 168 of the bore 16. This is possible because in a manner similar to the lower end of the insulator the beveled surface 37 on the upper end of the insulator together with the complementary surface 41 of the end cup 38 approximates a ball and socket joint, the radius of which, in this case as opposed to the lower end, is substantially equal to the length of the insulator 11. Accordingly, any radial displacement of the upper end 21 of the conductor 17 is impossible since the center point of the upper ball and socket joint is substantially at the opposite or lower end of the insulator which is the same point about which the overall insulator assembly forces the lower end 22 of the conductor 17 to move.

In the depicted bushing 10, the insulator, as previously mentioned, is substantially 26 inches long. The diameter of the cap 38 is substantially 3 inches and the beveled angles C and D are each substantially 3 degrees. With this arrangement, the lower bottom end 22 ofthe conductor 17 will be centered accurately to facilitate alignment with an associated device, while the upper end 21 will assume its position to allow greater camber in the bore 16 of the insulator without causing the conductor 17 to interfere with the wall surface of the bore 16.

For shunting the air gap 23 surrounding the conductor 17 in order to minimize the power factor, there is provided a shunting contact means 50. As shown in FIG. 2, the contact means in a preferred embodiment, includes a substantially square metallic sheet 51 having a plurality of hinged tongues 52. The arched edge portions 53 of the tongues 52 extend above the surface of plate 51 to form resilient deflectable contacts which are adapted to engage with the semiconductive glaze 15 fired into the wall of the insulator bore 16. In the preferred embodiment, the plate 51 is formed of a phosphorus bronze alloy which is spring tempered. An optimum structure, as herein depicted, utilizes two contact sheets 51 for shunting the air gap 23. As shown in FIG. 2, each sheet 51 is provided with seven horizontal rows of tongues 52 with the first and last horizontal rows having five tongues each, while intermediate horizontal rows each have six tongues 52. In limiting the first and last rows to only five tongues 52 each, room has been provided to accommodate the forming of screw receiving holes 56 and 57 at the upper and lower hand corners of the sheet 51, as viewed in FIG. 2. The right hand vertical edge of the sheet 51 is formed with 9 equispaced tongues 58. Thus, the contact sheet 51 presents a total of 49 contacts of tongues.

To assemble the bushing, two contact shunting sheets 51 are screw fastened to the conductor 17 and the sheets coiled about the conductor as shown in FIG. 1. The conductor 17 with the sheets 51 maintained in closely coiled relationship about the conductor is inserted into the bore 16 of the insulator. As each contact sheet 51 enters the bore 16, the sheet is released so that the inherent resiliency of the sheet operates to uncoil the sheet thereby moving the associated tongue edges 53 and 58 into contacting engagement with the semiconductive glaze 15 of the wall of the bore 16.

With the contact shunting sheets 51 located in the operative position, the bottom cap 27 is threaded onto the conductor 22 and into abutting engagement with the lower axial end face of the insulator 11. With this condition obtained, the lower end 22 of the conductor will be accurately centered with respect to the lower outer end 16A of the bore 16. This arrangement insures that the conductor through the contact shunting sheet 51 is not applying a radially acting stress to the insulator. With the conductor 17 positioned as described, the upper cap 38 is threaded on the upper end 21 of the conductor 17 and into an abutting engagement with the axial end face 37 of the insulator. The ball joint connection between the cap 38 and the outer axially end face 37 of the insulator will insure a proper seating of the cap on the insulator. As shown in FIG. 3, a padding washer 59 is interposed between the metallic cap 27 and the lower axial end face 25 of the porcelain insulator so that a grinding of the porcelain does not occur when tightening the cap 27 in position. A similar arrangement is provided for the upper end of the bushing. To this purpose a padding washer 61 is interposed between the top cap 38 and the upper axial end 37 of the insulator. Suitable means are provided to seal the assembly against the entrance of moisture, such as soft soldering the threads of joints.

In FIGS. 4 and 5 a modification of the contact sheets 51 is depicted. As shown, a contact sheet 71 of resilient metallic material is formed in a rectangular shape.

Three elongated slots 72, 73 and 74 are formed in the contact sheet. A plurality of score line 76, 77, 78, 79, 80 and 81 are providedin the surface of the sheet 71 to provide a control guide along which the plate is to bend when inserted into the bore 16 of the insulator 11. Thus, as depicted in FIG. 5, the plate 71 is bent along the score lines 76 to 81 inclusive to form contact edges 76A, 77A, 78A, 79A, 80A and 81A which engage the semiconductive glaze of the wall of the bore 16.

Spaced axially along the conductor 17A is a plurality of equispaced transverselyextending bores 84. The bores 84 are equal in number to the number of plates 71 to be utilized in the bushing 10A. Within each of the bores 84 there is provided a compression spring 86. An auxiliary or rivet contact 88 extends through the slot 73, which is disposed in the plate surface that is between the contact points 78A and 79A and into the bore 84. The head 88A of the rivet contact 88 is disposed between the surface of the contact plate 71 and the wall of the bore 16. In a similar manner, another rivet contact 89 extends through the aligned superimposed slots 72 and 74 into the bore 84. The head 89A of the rivet contact 89 is disposed between the surface of the contact plate 71 and the wall of the bore 16. Thus, when the contact plate 71 is in assembled position within the bore 16 of the insulator 11, the spring 86 applies a force to the adjacent surface of the contact plate 71 forcing the heads 88A and 89A to the rivet contacts into firm engagement with the semiconductive glaze surface of the wall of the bore.

By providing the elongated slots 72, 73 and 74 the inherent resiliency in the plate 71 allows the plate to expand within the bore 16 so that the contact edges 76A through 81A firmly engage the semiconductive glaze surface of the wall of the bore 16.

With the modified form of contact arrangement each element provides for eight contacts distributed annularly around the bore. Thus, eleven contact elements inserted into the bore 16 will give a total of 88 contacts which are substantially equal to the number of contacts that are obtained with the two contact sheets 51.

The embodiments of the invention in which an exclu- 1 sive property or privilege isclaimed are defined as follows:

1. In a dry type bushing;

an insulator having a first dielectric constant;

an axially extending bore having a wall surface formed in said insulator;

.a conductor extending through said bore but not in contact with the wall thereof;

an air gap defined by said conductor and the wall of said bore, the air in said air gap having a second dielectric constant which is lower than the first dielectric constant of said insulator;

a semiconductive glaze on the wall of said bore with shunting means having a multitude of connections between the conductor and the semiconductive glaze distributed both axially along the length of the bore of said insulator and angularly about the circumference of the wall surface of the bore of said insulator to shunt out the air of said air gap to thereby remove the air from the insulation system of said bushing, whereby the electrical stress is uniformly distributed and a lower value for the voltage gradient is obtained to thereby control the corona discharge between said conductor and the wall of said boref 2. In a dry type bushing:

an insulator body having an axial bore;

a coating of a semiconductive glaze on the wall of the bore of said insulator;

a conductor extending through the bore of said insulator but not in contact with the wall of the bore;

an air gap defined by the surface of said conductor and the wall of the bore of said insulator;

a conductive shunt of a relatively thin, resilient metallic sheet disposed in coiled axially extending relationship in the bore of said insulator, said metallic sheet being constructed'and arranged to present a multiplicity of outwardly extending contacts arranged in a plurality of annular rows distributed axially along the bore of said insulator to firmly engage the semiconductive glaze on the wall of the bore of said insulator; and

means electrically connecting said metallic sheet to said conductor;

whereby said metallic sheet shunts said air gap and the electrical field is thereby made more uniform to control the power factor and reduce the corona inception probability.

3. A dry type bushing according to claim 2 wherein said sheet is manufactured of a spring phosphorus bronze alloy, and said contacts are formed as punched tongues having one end attached to said sheet, said tongues being arranged in rows and constructed and arranged to extend radially outwardly into engagement with said semiconductive glaze when said sheet is mounted in coiled relationship with the bore of said insulator around said conductor.

4. In a dry type bushing:

an insulator body having an axial bore;

a coating of a semiconductive glaze on the wall of the bore of said insulator;

a conductor extending through the bore of said insulator but not in contact with the wall of the bore;

an air gap defined by the surface of said conductor and the wall of the bore of said insulator;

a conductive shunt of a relatively thin, resilient metallic sheet disposed and arranged in coiled axially extending relationship in the bore of said insulator, said metallic sheet being constructed and arranged to present a multiplicity of contacts which extend axially outwardly with respect to the axis of the bore of said insulator and in firm engagement with the semiconductive glaze on the wall of the bore of said insulator;

means electrically connecting said metallic sheet to said conductor comprising a springcarried by said conductor and constructed and arranged to engage and bias said metallic sheet in a direction to effect positive engagement of said extending contacts with the wall of the bore of said insulator','and

whereby said metallic sheet shunts said air gap and the electrical field is thereby made more uniform to control the power factor and reduce the corona inception probability.

5. A dry type bushing according to claim 4 wherein said spring is a compression spring disposed in said conductor and extending radially outwardly therefrom into biasing engagement with said metallic sheet;

an opening in said metallic sheet adjacent to said compression spring; and,

an axuiliary contact having a head portion and an elongated body portion, said body portion extending through the opening in said metallic sheet into the interior of said compression spring, said head portion of said auxiliary contact being disposed between said metallic sheet and the wall of said bore, whereby said compression spring operates to bias said metallic sheet in a direction to effect positive engagement of said outwardly extending contacts with the wall of said bore and also bias the head portion of said auxiliary contact into positive engagement with the wall of said bore.

A dry type bushing according to claim 4 wherein said metallic sheet is formed into a symmetrical configuration having substantially equal sides and presenting I external contact edges, the ends of said formed metallic sheet overlapping, with said overlapping ends having axially extending slots formed therein and disposed in coinciding overlapping relationship;

a third slot formed in said metallic sheet in a position diametrically opposite to said overlapping end slots;

bore formed in said conductor having an axis in a plane transverse to the axis of said conductor; compression spring disposed within said conductor bore and of a length so that its ends engage said metallic sheet, said metallic sheet being orientated in said insulator bore in a manner that said overlapping slots are adjacent to one end of said spring and said third slot is adjacent to the other end of said spring;

first auxiliary contact having a head portion and a body portion, said first auxiliary contact being disposed in a manner that the body portion thereof extends through said coinciding slots and into one end of the bore in said conductor, the head of said first auxiliary contact being disposed between said metallic sheet and the wall of said insulator bore; and, second auxiliary contact having a head portion and a body portion, said second auxiliary contact being disposed in a manner that the body portion thereof extends through said third slot and into the other end of said bore in said conductor, the head of said second auxiliary contact being disposed between said metallic sheet and the wall of said insulator bore, whereby said compression spring operates to apply a force to said metallic sheet to thereby forcefully engage the head portions of said auxiliary contacts with the wall of said insulator bore and the force applied to said metallic sheet by said spring operates also to urge said metallic sheets to expand to firmly engage said contact edges with the wall of said insulator bore.

ln a dry type bushing;

an insulator; an axially extending bore formed in said insulator;

a first end cap threadedly engaged on said first threaded end of said conductor and disposed in abutting securing engagement with an adjacent axial end of said insulator, the abutting surfaces of said insulator and said first end cap being constructed and arranged as complementary spherical surfaces which form a ball and socket joint the radii of which are substantially less than the length of said insulator to thereby effect positive radial positioning of the first end of said conductor with respect to the end opening of said bore through which said first end of said conductor extends; and,

a second end cap threadedly engaged on said second threaded end of said conductor and disposed in abutting securing engagement with an adjacent axial end of said insulator, the abutting surfaces of said insulator and said second end cap being constructed and arranged as complementary spherical surfaces which form a ball and socketjoint the radii of which are substantially equal to the length of said insulator thereby operating to allow said second end of said conductor to assume any radial position with respect to the end opening of said bore through which said second end of said conductor extends.

8. In a dry type bushing;

an insulator;

an axially extending bore formed in said insulator;

a conductor disposed within said bore and having first and second threaded ends extending outwardly of said bore;

a first end cap threadedly engaged on said first threaded end of said conductor and disposed in abutting engagement with the axial end of said insulator, the abutting surfaces of said axial end of said insulator and said first end cap being constructed and arranged as complementary conical surfaces having cone angles of substantially 12 degrees to thereby provide positive centering of said first threaded end of said conductor with respect to the end opening of said bore through which said first threaded end of said conductor extends; and,

a second end cap threadedly engaged on said second threaded end of said conductor and disposed in abutting engagement with the axial end of said insulator, the abutting surfaces of said axial end of said insulator and said second end cap being constructed and arranged as complementary conical surfaces having cone angles of substantially 3 degrees which cooperate to permit said second threaded end of said conductor to assume a radial position with respect to the end opening ofsaid bore through which said second threaded end of said conductor extends as dictated by the centering of said first threaded end of said conductor within the opening of the bore through which said first threaded end extends. 

1. In a dry type bushing; an insulator having a first dielectric constant; an axially extending bore having a wall surface formed in said insulator; a conductor extending through said bore but not in contact with the wall thereof; an air gap defined by said conductor and the wall of said bore, the air in said air gap having a second dielectric constant which is lower than the first dielectric constant of said insulator; a semiconductive glaze on the wall of said bore with shunting means having a multitude of connections between the conductor and the semiconductive glaze distributed both axially along the length of the bore of said insulator and angularly about the circumference of the wall surface of the bore of said insulator to shunt out the air of said air gap to thereby remove the air from the insulation system of said bushing, whereby the electrical stress is uniformly distributed and a lower value for the voltage gradient is obtained to thereby control the corona discharge between said conductor and the wall of said bore.
 2. In a dry type bushing: an insulator body having an axial bore; a coating of a semiconductive glaze on the wall of the bore of said insulator; a conductor extending through the bore of said insulator but not in contact with the wall of the bore; an air gap defined by the surface of said conductor and the wall of the bore of said insulator; a conductive shunt of a relatively thin, resilient metallic sheet disposed in coiled axially extending relationship in the bore of said insulator, said metallic sheet being constructed and arranged to present a multiplicity of outwardly extending contacts arranged in a plurality of annular rows distributed axially along the bore of said insulator to firmly engage the semiconductive glaze on the wall of the bore of said insulator; and means electrically connecting said metallic sheet to said conductor; whereby said metallic sheet shunts said air gap and the electrical field is thereby made more uniform to control the power factor and reduce the corona inception probability.
 3. A dry type bushing according to claim 2 wherein said sheet is manufactured of a spring phosphorus bronze alloy, and said contacts are formed as punched tongues having one end attached to said sheet, said tongues being arranged in rows and constructed and arranged to extend radially outwardly into engagement with said semiconductive glaze when said sheet is mounted in coiled relationship with the bore of said insulator around said conductor.
 4. In a dry type bushing: an insulator body having an axial bore; a coating of a semiconductive glaze on the wall of the bore of said insulator; a conductor extending through the bore of said insulator but not in contact with the wall of the bore; an air gap defined by the surface of said conductor and the wall of the bore of said insulator; a conductive shunt of a relatively thin, resilient metallic sheet disposed and arranged in coiled axially extending relationship in the bore of said insulator, said metallic sheet being constructed and arranged to present a multiplicity of contacts which extend axially outwardly with respect to the axis of the bore of said insulator and in firm engagement with the semiconductive glaze on the wall of the bore of said insulator; means electrically connecting said metallic sheet to said conductor comprising a spring carried by said conductor and constructed and arranged to engage and bias said metallic sheet in a direction to effect positive engagement of said extending contacts with the wall of the bore of said insulator; and whereby said metallic sheet shunts said air gap and the electrIcal field is thereby made more uniform to control the power factor and reduce the corona inception probability.
 5. A dry type bushing according to claim 4 wherein said spring is a compression spring disposed in said conductor and extending radially outwardly therefrom into biasing engagement with said metallic sheet; an opening in said metallic sheet adjacent to said compression spring; and, an auxiliary contact having a head portion and an elongated body portion, said body portion extending through the opening in said metallic sheet into the interior of said compression spring, said head portion of said auxiliary contact being disposed between said metallic sheet and the wall of said bore, whereby said compression spring operates to bias said metallic sheet in a direction to effect positive engagement of said outwardly extending contacts with the wall of said bore and also bias the head portion of said auxiliary contact into positive engagement with the wall of said bore.
 6. A dry type bushing according to claim 4 wherein said metallic sheet is formed into a symmetrical configuration having substantially equal sides and presenting external contact edges, the ends of said formed metallic sheet overlapping, with said overlapping ends having axially extending slots formed therein and disposed in coinciding overlapping relationship; a third slot formed in said metallic sheet in a position diametrically opposite to said overlapping end slots; a bore formed in said conductor having an axis in a plane transverse to the axis of said conductor; a compression spring disposed within said conductor bore and of a length so that its ends engage said metallic sheet, said metallic sheet being orientated in said insulator bore in a manner that said overlapping slots are adjacent to one end of said spring and said third slot is adjacent to the other end of said spring; a first auxiliary contact having a head portion and a body portion, said first auxiliary contact being disposed in a manner that the body portion thereof extends through said coinciding slots and into one end of the bore in said conductor, the head of said first auxiliary contact being disposed between said metallic sheet and the wall of said insulator bore; and, a second auxiliary contact having a head portion and a body portion, said second auxiliary contact being disposed in a manner that the body portion thereof extends through said third slot and into the other end of said bore in said conductor, the head of said second auxiliary contact being disposed between said metallic sheet and the wall of said insulator bore, whereby said compression spring operates to apply a force to said metallic sheet to thereby forcefully engage the head portions of said auxiliary contacts with the wall of said insulator bore and the force applied to said metallic sheet by said spring operates also to urge said metallic sheets to expand to firmly engage said contact edges with the wall of said insulator bore.
 7. In a dry type bushing; an insulator; an axially extending bore formed in said insulator; a conductor disposed within said bore and having first and second threaded ends extending outwardly from said bore; a first end cap threadedly engaged on said first threaded end of said conductor and disposed in abutting securing engagement with an adjacent axial end of said insulator, the abutting surfaces of said insulator and said first end cap being constructed and arranged as complementary spherical surfaces which form a ball and socket joint the radii of which are substantially less than the length of said insulator to thereby effect positive radial positioning of the first end of said conductor with respect to the end opening of said bore through which said first end of said conductor extends; and, a second end cap threadedly engaged on said second threaded end of said conductor and disposed in abutting securing engagement with an adjacent axial end of said insulator, the Abutting surfaces of said insulator and said second end cap being constructed and arranged as complementary spherical surfaces which form a ball and socket joint the radii of which are substantially equal to the length of said insulator thereby operating to allow said second end of said conductor to assume any radial position with respect to the end opening of said bore through which said second end of said conductor extends.
 8. In a dry type bushing; an insulator; an axially extending bore formed in said insulator; a conductor disposed within said bore and having first and second threaded ends extending outwardly of said bore; a first end cap threadedly engaged on said first threaded end of said conductor and disposed in abutting engagement with the axial end of said insulator, the abutting surfaces of said axial end of said insulator and said first end cap being constructed and arranged as complementary conical surfaces having cone angles of substantially 12 degrees to thereby provide positive centering of said first threaded end of said conductor with respect to the end opening of said bore through which said first threaded end of said conductor extends; and, a second end cap threadedly engaged on said second threaded end of said conductor and disposed in abutting engagement with the axial end of said insulator, the abutting surfaces of said axial end of said insulator and said second end cap being constructed and arranged as complementary conical surfaces having cone angles of substantially 3 degrees which cooperate to permit said second threaded end of said conductor to assume a radial position with respect to the end opening of said bore through which said second threaded end of said conductor extends as dictated by the centering of said first threaded end of said conductor within the opening of the bore through which said first threaded end extends. 